Alkylation is a process technology in which low molecular weight olefins, too volatile to be used in gasoline, can be combined with an isoparaffin to make high octane motor fuel. The isoparaffin commonly alkylated is isobutane although isopentane could be utilized if desired. The most commonly used olefins are propylene and butylenes with amylenes being used less frequently. As used herein, the term butylenes refers to 1-butene, cis-2-butene, trans-2-butene and isobutylene. An acid catalyst such as hydrofluoric (HF) or sulfuric serves to catalyze the reaction. The invention will be described hereinafter in terms of the use of HF acid but is not limited to such use.
Although HF alkylation is considered by many to be a mature refining technology, many new plants are being constructed. The phase-down of lead as an octane enhancer has renewed a demand for processes such as HF alkylation which offers cost effective octane enhancement.
In a well proven alkylation process isobutane is catalytically alkylated with butylene using liquid hydroflouric acid as the catalyst, to produce a high octane motor gasoline product, or a light alkylate suitable for aviation gasoline blending. In this process the saturated isobutane feed combined with the olefin feed, usually obtained from a catalytic cracking unit, passes through feed driers. Downstream of the driers the feed mixture, olefin and isobutane, is combined with recycle isobutane. This mixture then flows to the alkylation reactor. In the reactor, the hydrocarbon feed mixture is highly dispersed into a moving bed of liquid HF catalyst where product alkylate is formed.
From the reaction zone, the hydrocarbon components and the catalyst flow upward to a settling zone. Here the catalyst breaks out as a bottom phase and flows by gravity through a cooler back to the reaction zone, where the previous cycle is repeated. The hydrocarbon phase from the settling zone, containing propane, isobutane, normal butane and alkylate, is charged to a fractionation system where the products including alkylate, propane and normal butane are separated. Isobutane is recycled to the alkylation reactor.
Generally, the octane number of the alkylate is the criterion utilized to judge the quality of the product from the alkylation process. It is known in the art that several parameters influence this octane number. Reaction temperature, acid to hydrocarbon ratio, stream impurities and isoparaffin-to-olefin ratio are all important parameters. The change in any of these parameters can affect the octane-number of the alkylate produced.
In order to maintain a desired isoparaffin-to-olefin ratio it has been proposed to analyze on line each component of the feed stream comprising fresh isobutane, recycle isobutane and olefin which typically forms the combined feed stream to the reactor, and/or to analyze an individual fresh isobutane stream and an olefin stream prior to the combining of the streams. A ratio controller is then provided to adjust the flow of one or more of the feed streams in response to the analysis ratio so that the isoparafin and olefins supplied to the reactor can be maintained at a desired ratio.
While the above described control method which manipulates the flow of one or more of the individual feed streams in response to the measured analysis ratio has proved effective for controlling the isobutane-to-olefin ratio under normal conditions, it is subject to certain limitations. For example, several parameters, such as isobutane and olefin content of fresh olefin feed, composition of the recycle isobutane stream, and volume flow rates of these streams can vary significantly and at random times. Also, the operation of the driers in the mixing section can disturb the isobutane-to-olefin ratio. These multiple disturbances can affect the operation of a controller that manipulates a relatively low volume feed stream flowing an individual feed e.g. isobutane to the alkylation reactor, such that the actual isobutane-to-olefin ratio varies as much as .+-.15 percent around its set point. In addition, if significant changes in the isoparaffin feed flow to the alkylation reactor are required to maintain a desired isoparaffin-to-olefin ratio, correspondingly significant changes in the liquid level within the reactor will occur.